Quantitative microbial risk assessment of the impact of drought and seasonality on a de facto reuse system in Southern Nevada, USA

De facto reuse is already part of many drinking water supplies in arid regions: treated wastewater is discharged to rivers or reservoirs that later serve as drinking water sources. As drought reduces available supplies, the percentage of recycled water in these source waters can rise, bringing renewed attention to a straightforward question from the public and regulators alike: how does changing drought and seasonality affect pathogen risk at the tap?

A recent peer-reviewed study in Environmental Science: Water Research & Technology addresses that question for Southern Nevada’s Lake Mead system using quantitative microbial risk assessment (QMRA) to estimate infection risk under different lake levels and seasonal conditions. The paper, co-authored by Carollo’s Emily Clements and a broader research team, offers a clear, data-driven look at how a real-world multi-barrier system performs under stress.

What De Facto Reuse Means for Drinking Water in Drought Conditions

De facto reuse (DFR) is the “incidental or unintentional incorporation of treated wastewater into natural water bodies used as a source of drinking water.” In Southern Nevada, this is especially relevant because the Las Vegas area relies heavily on Lake Mead for drinking water, while treated effluent from multiple wastewater treatment plants is discharged into the lake via the Las Vegas Wash. During drought, the factors that shape risk can shift in ways that matter: dilution can decrease and travel times in the reservoir can shorten as lake levels fall, reducing the time available for natural pathogen die-off.

Using QMRA to Estimate Pathogen Risk from Source to Tap

To evaluate risk through this full system, the researchers modeled infection risks for five pathogens commonly considered in potable reuse risk work: norovirus, adenovirus, enterovirus, Cryptosporidium, and Giardia. This study started with observed raw wastewater pathogen concentrations from local facilities, then incorporated facility-specific wastewater treatment performance (log reduction values, or LRVs), dilution and decay in the Las Vegas Wash, dilution and decay in Lake Mead informed by hydrodynamic modeling, and engineered drinking water treatment at two treatment plants.

What the Lake Mead Scenarios Showed at Lower Water Levels

To capture drought impacts, the researchers modeled three lake elevations: 329 m as a baseline condition, plus 312 m and 297 m to represent potential continued drought scenarios. The results showed that risk increased as lake levels dropped, largely because lower water levels can reduce reservoir travel time and therefore reduce natural attenuation. Even so, the study found the overall system remained protective of public health.

Why Seasonality Matters Even in a Highly Managed System

The study found that seasonal patterns affected risk estimates because seasonality can influence wastewater pathogen concentrations, lake stratification and mixing, travel time from discharge points to the drinking water intake, and the percent DFR at the intake. In other words, higher risk seasons aren’t driven only by higher pathogen loads; they can also be driven by faster transport and reduced die-off under certain hydrodynamic conditions.

A Real-World Look at the Value of Multiple Barriers

DFR employs a system of barriers that includes engineered treatment and natural attenuation working together. The authors conclude that the current design and operation of Southern Nevada’s DFR system is protective of public health for enteric pathogen exposure, even if the Colorado River Basin drought continues or worsens.

Read the full article to explore the modeling approach, lake-level scenarios, and seasonal findings in more detail.